A palm vein authentication captures a palm vein pattern by a camera using near-infrared light, and performs a personal authentication by matching the captured palm vein pattern to a registered vein pattern. The palm vein authentication is an example of a biometric authentication technique. The palm vein authentication can perform a noncontact biometric authentication.
When registering the vein pattern, it is possible in many cases to capture the palm vein pattern in a state in which the palm is stably supported on a guide member. On the other hand, when authenticating the vein pattern, the palm vein pattern may be captured in an environment in which no guide member is provided. For example, compared to the time when the vein pattern is registered, the palm at the time when authenticating the vein pattern may be closed or excessively open.
Even in a case in which the authentication is performed in the environment in which the guide member is provided, a user may not necessarily place the user's palm on the guide member each time at the same orientation (or posture). In such a case, compared to the time when the vein pattern is registered, the palm at the time when authenticating the vein pattern may be closed or excessively open, for example.
When the orientation of the palm at the time of the registration and the orientation of the palm at the time of the authentication differ, an authentication error may cause a false rejection to be generated. For this reason, at the time of the authentication, it is desirable to normalize a three-dimensional shape of the captured palm.
Conventionally, there is a proposed method that normalizes the palm vein pattern by correcting an inclination or a rotation of the palm at the time of the authentication. However, even when the palm vein pattern is normalized by correcting the inclination or the rotation of the palm, it is difficult to normalize the palm vein pattern by correcting, with a high accuracy, a distortion in the three-dimensional shape of the palm that is captured at the time of the authentication, in the case in which the palm is closed or excessively open compared to the palm at the time of the registration. In addition, when an exclusive algorithm is employed to normalize the palm vein pattern by correcting the closed or excessively open palm in order to cope with the palm that is closed or excessively open compared to the palm at the time of the registration, the algorithm becomes complex. However, when the algorithm is complex, it is difficult to correct, at a high speed, the distortion in the three-dimensional shape of the palm that is closed or excessively open and is captured at the time of the authentication. On the other hand, when the algorithm is simplified in order to enable a high-speed normalization process, it is difficult to normalize the palm vein pattern by correcting, with a high accuracy, the distortion in the three-dimensional shape of the palm that is closed or excessively open and is captured at the time of the authentication.
In a case in which a target object is captured using illumination light or natural light other than the near-infrared light, a distortion may be generated in the three-dimensional shape of the captured target object depending on the orientation (or posture) of the target object. Accordingly, even in such a case, it is desirable to normalize the image of the target object by correcting the distortion in the three-dimensional shape of the captured target object, similarly to the above described case in which the palm vein pattern is captured. Examples of capturing the target object using the illumination light other than the near-infrared light includes capturing a palm-print pattern, and capturing a palm-reading pattern (or lines of the palm), for example.
A technique for approximating the three-dimensional shape by a quadric surface, and a SFS (Shape From Shading) technique for recognizing the three-dimensional shape of the target object from a luminance distribution of the image of the target object captured by illuminating the target object by the illumination light are proposed in Japanese Laid-Open Patent Publications No. 2007-213199 and No. 2015-170322, for example.
Conventionally, it is difficult to normalize the palm vein pattern by correcting, with a high accuracy, the distortion in the three-dimensional shape of the palm that is closed or excessively open and is captured at the time of the authentication.